This invention relates to data acquisition systems and methods.
Data acquisition systems and methods may be used in a variety of applications. For example, data acquisition techniques may be used in nuclear magnetic resonance imaging systems and Fourier transform spectrometer systems. Such techniques also may be used in mass spectrometer systems, which may be configured to determine the concentrations of various molecules in a sample. A mass spectrometer operates by ionizing electrically neutral molecules in the sample and directing the ionized molecules toward an ion detector. In response to applied electric and magnetic fields, the ionized molecules become spatially separated along the flight path to the ion detector in accordance with their mass-to-charge ratios.
Mass spectrometers may employ a variety of techniques to distinguish ions based on their mass-to-charge ratios. For example, magnetic sector mass spectrometers separate ions of equal energy based on their momentum changes in a magnetic field. Quadrupole mass spectrometers separate ions based on their paths in a high frequency electromagnetic field. Ion cyclotrons and ion trap mass spectrometers distinguish ions based on the frequencies of their resonant motions or stabilities of their paths in alternating voltage fields. Time-of-flight (or xe2x80x9cTOFxe2x80x9d) mass spectrometers discriminate ions based on the velocities of ions of equal energy as they travel over a fixed distance to a detector.
In a time-of-flight mass spectrometer, neutral molecules of a sample are ionized, and a packet (or bundle) of ions is synchronously extracted with a short voltage pulse. The ions within the ion source extraction are accelerated to a constant energy and then are directed along a field-free region of the spectrometer. As the ions drift down the field-free region, they separate from one another based on their respective velocities. In response to each ion packet received, the detector produces a data signal (or transient) from which the quantities and mass-to-charge ratios of ions contained in the ion packet may be determined. In particular, the times of flight between extraction and detection may be used to determine the mass-to-charge ratios of the detected ions, and the magnitudes of the peaks in each transient may be used to determine the number of ions of each mass-to-charge in the transient.
A data acquisition system (e.g., an integrating transient recorder) may be used to capture information about each ion source extraction. In one such system, successive transients are sampled and the samples are summed to produce a summation, which may be transformed directly into an ion intensity versus mass-to-charge ratio plot, which is commonly referred to as a spectrum. Typically, ion packets travel through a time-of-flight spectrometer in a short time (e.g., 100 microseconds) and ten thousand or more spectra may be summed to achieve a spectrum with a desired signal-to-noise ratio and a desired dynamic range. Consequently, desirable time-of-flight mass spectrometer systems include data acquisition systems that operate at a high processing frequency and have a high dynamic range.
In one data acquisition method, which has been used in high-speed digital-to-analog converters, data is accumulated in two or more parallel processing channels (or paths) to achieve a high processing frequency (e.g., greater than 100 MHz). In accordance with this method, successive samples of a waveform (or transient) are directed sequentially to each of a set of two or more processing channels. The operating frequency of the components of each processing channel may be reduced from the sampling frequency by a factor of N, where N is the number of processing channels. The processing results may be stored or combined into a sequential data stream at the original sampling rate.
When applied to applications in which sample sets (or transients) are accumulated to build up a composite signal (e.g., TOF mass spectrometer applications), the process of accumulating samples in parallel processing channels may introduce noise artifacts that are not reduced by summing the samples from each processing channel. In particular, although contributions from random noise and shot noise may be reduced by increasing the number of transients summed, each processing channel may contribute to the composite signal a non-random pattern noise that increases with the number of transients summed. Such pattern noise may result from minute differences in digital noise signatures induced in the system by the different parallel processing paths. For example, the physical separations between the components (e.g., discrete memory, adders and control logic) of a multi-path or parallel-channel data acquisition system may generate voltage and current transitions within the board or chip on which the data acquisition system is implemented. The unique arrangement of each processing path may induce a unique digital noise signature (or pattern noise) in the analog portion of the system. The resulting digital noise signature increases as the composite signal is accumulated, limiting the ability to resolve low-level transient signals in the composite signal.
The invention features improved data acquisition systems and methods that substantially reduce accumulated pattern noise to enable large numbers of data samples to be accumulated rapidly with low noise and high resolution.
In one aspect of the invention, a data acquisition system includes a sampler and an accumulator. The sampler is configured to produce a plurality of data samples from a transient sequence in response to a sampling clock. The accumulator is coupled to the sampler and is configured to accumulate data samples in response to an accumulation clock that is shifted in phase relative to the sampling clock.
Embodiments may include one or more of the following features.
The accumulator preferably is configured to accumulate corresponding data samples across the transient sequence (i.e., data samples from different transients having similar mass-to-charge ratios are summed together to produce a spectrum).
The accumulation clock may be shifted between 90xc2x0 and 270xc2x0 relative to the sampling clock, and preferably is shifted approximately 180xc2x0 relative to the sampling clock. The data acquisition system may include a multiphase frequency synthesizer that is configured to generate the sampling clock and the accumulation clock.
In one embodiment, the accumulator comprises two or more parallel accumulation paths and accumulates corresponding data samples across the transient sequence through different accumulation paths. Each accumulation path preferably accumulates data samples in response to a respective accumulation clock. The phase of the accumulation clock for each accumulation path may be shifted relative to the sampling clock by a respective amount. A controller preferably is coupled to the accumulator and is configured to cycle the accumulation of data samples through each of the accumulation paths.
In another aspect, the invention features a time-of-flight mass spectrometer that includes an ion detector, a sampler, and an accumulator. The ion detector is configured to produce a transient sequence from a plurality of respective ion packets. The sampler is configured to produce a plurality of data samples from the transient sequence in response to a sampling clock. The accumulator is coupled to the sampler and is configured to accumulate corresponding data samples across the transient sequence in response to an accumulation clock that is shifted in phase relative to the sampling clock.
In another aspect, the invention features a method of acquiring data. In accordance with this inventive method, a plurality of data samples is produced from a transient sequence in response to sampling clock, and corresponding data samples across the transient sequence are accumulated in response to an accumulation clock that is shifted in phase relative to the sampling clock.
The phase of the accumulation clock preferably is shifted relative to the sampling clock by an amount selected to reduce noise in an accumulator output signal. Corresponding data samples preferably are accumulated across the transient sequence through two or more parallel accumulation paths. Data samples preferably are accumulated through each accumulation path in response to a respective accumulation clock. The phase of each accumulation path clock preferably is shifted to reduce noise in the accumulated data samples. The accumulation of data samples preferably is cycled through each of the parallel accumulation paths.
Among the advantages of the invention are the following.
By shifting the accumulation clock relative to the sampling clock, the overall noise level induced in the spectrum data by the accumulator may be reduced. This feature improves the signal-to-noise ratio in the resulting spectrum and, ultimately, improves the sensitivity of the data acquisition system.